1. Field of the Invention
The present invention relates generally to a semi-closed circuit passive gas addition breathing apparatus and more particularly to a variable volume ratio compound counterlung used in a rebreathing apparatus.
2. Description of Related Art
Conventional semi-closed rebreathers operate by delivering a premixed gas from a scuba cylinder through a constant flow regulating device, usually by supplying a regulated gas supply to a changeable orifice. Gas is delivered at a preset rate regardless of depth. The gas being breathed is recirculated, and as the oxygen within the mixture is metabolically consumed, it is hopefully being adequately replaced on a continuous basis with a predetermined continuous flow of oxygen enriched gas.
Rebreathers consist of a breathing loop from which the diver inhales and into which the diver exhales. As most of the exhaled gas stays in the breathing loop, rebreathers allow for much greater gas efficiency than open circuit systems. This greater gas efficiency allows for longer duration dives as compared to open circuit systems, or, conversely, requires less gas supply for a dive of equal duration.
The breathing loop generally includes a relief valve, scrubber, counterlung, depth equalization regulator, continuous injection system, hoses and a mouthpiece. The relief valve is utilized for dumping or venting excess gas in the breathing loop created by the rebreather on ascent and excess gas which is produced with the use of constant (active) addition systems. The scrubber cleanses the exhaled gas of carbon dioxide. The counterlung or breathing bag allows for the retention of the diver's exhalation gas. The injection system adds fresh gas to the carbon dioxide cleansed gas in the breathing loop. The depth equalization regulator adds supply mix to the loop to keep pace with depth increases. The hoses are utilized to connect the counterlung and scrubber with the mouthpiece. The mouthpiece is connected to the two hoses and is the point on the breathing loop where the diver exhales and inhales. Typically, two conventional one-way valves are incorporated into the mouthpiece.
Rebreathers normally include a harness to strap the unit to the diver, with some units also including a protective case for the various above described components.
As stated above, rebreathers generally work by recycling most of a diver's exhaled breath, which travels through the breathing loop through the scrubber, and is returned to the diver during inhalation. The use of a rebreather allows a diver to remain underwater for a relatively long time as compared to the use of open circuit equipment.
Accordingly, rebreathers allow exhaled gas to be cleansed of carbon dioxide and replenished with fresh oxygen for further consumption. A traditional fixed flow (active addition) semi-closed rebreather recycles the gas the diver is breathing, removing excess carbon dioxide from the exhaled gas and replacing it with a measured amount of premixed gas to maintain an oxygen partial pressure in the inspired gas that will continue to support metabolism.
There are several previously known types of operating systems for semi-closed circuit rebreathers, including fixed discharge ratio, continuous injection and mechanically pulsed. In the 1970's, as electronically controlled rebreathers were coming into their own, a fixed discharge ratio counterlung (an inner bellows within an outer bellows) was developed for semi-closed use in Europe. This type of rebreather was coined the first "passive" addition or counter mass ratio system. "Passive" means gas is only added as required to replace gas that has been discharged from the breathing loop by the control mechanism.
The "passive" addition system discharged a fixed percentage of each exhalation overboard, thus responding to respiratory minute volume ("RMV") or work rate. As such, reasonably tight decompression schedules could be computed for semi-closed equipment, eliminating the need for complex electronic oxygen monitoring.
Any system keyed to RMV is essentially using the diver as a sensor. The passive system uses a proportional discharge valve or a bellows within a bellows to discharge a fixed percentage of every exhalation overboard. The missing part of the exhalation is made up "passively" by one or two demand regulators on the following inhalation. Excess gas in the breathing loop from reduced ambient pressure is vented off by an overpressure relief valve. The fixed discharged ratio units maintain reasonably steady oxygen fractions in the breathing loop. The counterlung does not have to be purged on normal ascents to prevent hypoxia.
One drawback with the fixed discharged ratio semi-closed circuit is that it is not as gas efficient as electronic closed circuit rebreathers or constant flow (active) semi-closed rebreathers due to the fact that gas usage increased with depth similar to open circuit equipment. Furthermore, different diver positions often caused gas to be lost. The increased gas usage limits dive duration at depth as compared to other types of semi-closed units. Thus, despite solving decompression problems the bellow within a bellow system was ultimately abandoned due to its limited dive duration capabilities.
The continuous injection system is an active addition system and typically bleeds a fixed flow of single source mixed gas into the breathing loop from a variable or changeable fixed orifice. The flow rate is determined by estimating the diver's work rate for the intended dive and hopefully ensuring that enough oxygen from the mixed gas supply enters the system to meet anticipated metabolic requirements. Hypoxia is possible if the counterlung is not purged during ascent. Additionally, extended periods of higher than anticipated work loads can also produce hypoxia.
The mechanically pulsed semi-closed rebreather is also an active system and uses a bellows counterlung to mechanically drive a ratchet/cam that pulses gas addition valves in approximate response to respiratory minute volume. The gas addition is from a single mixed gas supply and is regulated to provide reasonably tight oxygen fractions in the breathing loop. Excess gas in the breathing loop from additions or reduced ambient pressure is vented off by an overpressure relief valve. However, with this type of unit, there are more single point addition failure possibilities.
Accordingly, no prior RMV controlled recirculating breathing system has incorporated a mass-constant discharge capability. Thus, there exists a need for a "passive" gas addition semi-closed circuit rebreather unit which provides for a variable discharge ratio which changes with depth to effect a mass constant discharge ratio (to reduce gas wastage) that is controlled by the diver's RMV (to make the unit responsive to actual metabolic requirements). It is therefore, to the effective resolution of the aforementioned problems and shortcomings that the present invention is directed.